Circuit and method for controlling current flow through a solenoid

ABSTRACT

A solenoid control circuit that includes two switching networks that are selectively opened and closed in a current limit cycle to control the magnitude of the current flowing through the solenoid. The configuration of the switching networks is such that current flows through an energy dissipating resistor only when the solenoid is commanded off by the solenoid control circuit, and not during the current limit cycle. Thus, the energy dissipated by this resistor is reduced relative to other solenoid control circuits, making it more efficient, and reducing EMI emissions from the circuit while providing for a fast solenoid response.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a solenoid control circuit and,more particularly, to a circuit and method for controlling the currentflow through a solenoid while simultaneously enhancing the solenoidresponse time.

[0002] Solenoids are used in varying industries and for variousapplications within these varying industries. For example, in theaerospace industry, solenoids are sometimes used in valves and actuatorsto move a component. As is generally known, solenoids are constructedusing inductive components, which may have a relatively slow responsetime, from both an activation and a deactivation standpoint. Thus, nomatter the particular industry and application, in some instances it maybe desirable to speed up the solenoid's response time.

[0003] Two methods may be employed to speed up a solenoid's activationtime. In particular, there is one way to speed up the activationresponse time and another way to speed up the deactivation responsetime. To speed up the activation response time, a relatively highsolenoid activation voltage is used. Thus, when the solenoid isactivated, this relatively high voltage is applied to the solenoid,which speeds up its activation response time. To speed up thedeactivation time, a high power deactivation circuit, such as a lowresistance, high power resistor, is used to quickly remove the storedenergy from the solenoid.

[0004] If a relatively high solenoid activation voltage is used, currentlevels through the solenoid coil and other circuit components will besubstantial. Thus, to keep the current flow to a level that does notdamage the circuit components, a current limit circuit may beincorporated. This current limit circuit may be periodically switched onand off, as necessary, to limit the magnitude of the current flow. Onedrawback of this particular arrangement is that when the current limitcircuit is activated, current may flow through the high powerdeactivation circuitry. Since the current limit circuit may be switchedon and off at a relatively high frequency, unnecessary power dissipationmay occur. Thus, large and bulky high-power components may be needed,high electromagnetic interference (EMI) emissions may occur, and theoverall efficiency of the solenoid driver circuit may be reduced, whilethe expense and weight of the circuit may be increased.

[0005] Hence, there is a need for a solenoid driver circuit thatenhances the response time of a solenoid, while addressing one or moreof the above-noted drawbacks. Namely, a circuit that does not result inunnecessarily high power dissipation, and/or does not use large andbulky, high power components, and/or does not cause relatively high EMIemissions, and/or has improved efficiency. The present inventionaddresses one or more of these needs.

SUMMARY OF THE INVENTION

[0006] The present invention provides a solenoid control circuit andmethod in which two switching networks are selectively opened and closedto control the magnitude of the current flowing through the solenoid.The configuration of the switching networks is such that current flowsthrough an energy dissipating resistor only when the solenoid iscommanded off by the solenoid control circuit, and not during thecurrent limit cycle of the control circuit. Thus, the energy dissipatedby this resistor is reduced relative to other solenoid driver circuits,thereby making it more efficient and reducing EMI emissions from thecircuit.

[0007] In one embodiment of the present invention, and by way of exampleonly, a circuit for controlling current flow through a solenoid includesa first controllable switch, a second controllable switch, a diode, anda controller circuit. The first controllable switch is electricallycoupled in parallel with a first resistive element. The secondcontrollable switch is electrically coupled in series with a secondresistor. The diode is electrically coupled in series between the firstcontrollable switch and the second controllable switch. The controllercircuit is operable to selectively open and close the first controllableswitch and the second controllable switch in response to a commandsignal, and to selectively open and close the second controllable switchbased on a magnitude of current flow through the solenoid.

[0008] In another exemplary embodiment, in a circuit including a seriesconnected first controllable switch and a diode that are electricallycoupled in parallel with a solenoid, and a series-connected secondcontrollable switch and first resistive element that are electricallycoupled in series with the solenoid, a method of controlling currentflow through the solenoid includes closing the first and secondcontrollable switches, whereby a current flows from a first voltagepotential through the solenoid, the second controllable switch, and thefirst resistive element. When the current flow through the solenoidreaches a first predetermined current magnitude, the second controllableswitch is opened, whereby the current flows through the solenoid, thediode, and the first controllable switch.

[0009] Other independent features and advantages of the preferredsolenoid control circuit will become apparent from the followingdetailed description, taken in conjunction with the accompanyingdrawings which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a simplified block diagram of a solenoid control circuitaccording to an exemplary embodiment of the present invention;

[0011]FIG. 2 is a detailed schematic diagram of a particular preferredembodiment of the present invention; and

[0012]FIG. 3 depicts various waveforms associated with the circuitdepicted in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] A simplified block diagram of a solenoid control circuitaccording to an exemplary embodiment of the present invention isdepicted in FIG. 1. The solenoid control circuit 100 is used to controlthe current flow through one or more solenoid coils 102, and includes afirst controllable switching network 104, a second controllableswitching network 106, and a controller circuit 108.

[0014] The first controllable switching network 104 is electricallyconnected in parallel with the solenoid coils 102, and includes a firstcontrollable switch 110, a first resistive element 112, and a diode 114.The first controllable switch 110 and first resistive element 112 areelectrically connected in parallel with one another. The diode 114 iselectrically connected in series with the first controllable switch 110and first resistive element 112. In particular, in the depictedembodiment the cathode of the diode 114 is electrically connected to thefirst controllable switch 110 and first resistive element 112. It willbe appreciated that the anode of the diode 114 could instead beelectrically connected to the first controllable switch 110 and firstresistive element 112, so long as the circuit operating voltages providethe correct biasing to achieve proper circuit operation. The firstcontrollable switching network 104 is also electrically connected to afirst circuit terminal 116.

[0015] When the solenoid control circuit 100 is energized and operable,the first circuit terminal 116 is used to couple the first switchingnetwork 102 to a first voltage potential 118. In the depictedembodiment, this first voltage potential 118 is a relatively highmagnitude, positive (relative to reference potential) voltage potential,which is used to enhance the turn on response of the solenoid coils 102.It is to be appreciated that the first voltage potential 118 may also bea relatively high magnitude, negative voltage potential, so long as thepolarity sensitive components of the solenoid control circuit 100 areappropriately connected to achieve proper circuit operation andresponse. Additionally, in the depicted embodiment, the first resistiveelement 112 is a resistor, though it will be appreciated that thiselement could also be a zener diode or other known resistive element.

[0016] The second controllable switching network 106 is electricallyconnected in series with the first controllable switching network 104and the solenoid coils 102, and includes a second controllable switch120 and a second resistor 122. The second controllable switching network106 is also electrically connected to a second circuit terminal 124.Similar to the first circuit terminal 116, when the solenoid circuit 100is energized and operable, the second circuit terminal 124 is used tocouple the second switching network 106 to a second voltage potential126. In the depicted embodiment, the second voltage potential 126 is aground or reference potential. It will also be appreciated that thesecond voltage potential 126 may also be a negative potential (relativeto reference potential) or a positive potential, so long as the polaritysensitive components of the solenoid control circuit 100 areappropriately connected to achieve proper circuit operation.

[0017] The controller circuit 108 includes at least two input ports, afirst input port 128 and a second input port 130. The first input port128 is used to receive a command signal for either energizing orde-energizing the solenoid coils 102. The second input port 130 iselectrically connected to the second switching network 106, between thesecond controllable switch 120 and the second resistor 122. Thus, thesecond input port 130 receives a feedback signal having a voltagemagnitude that is representative of the current flow through thesolenoid coils 102. Based on the signals received by the first 128 andsecond 130 input ports, the controller circuit 108 selectively opens andcloses the first 110 and second 120 controllable switches to energize,and appropriately limit current flow through, the solenoid coils 102. Amore detailed description of this functionality will now be provided.

[0018] If the first input port 128 receives the appropriate commandsignal for energizing the solenoid coils 102, then the controllercircuit 108 will cause both the first and second 120 controllableswitches to close. As a result, current will flow from the first voltagepotential 118, through the solenoid coils 102 and the second switchingnetwork 106, to the second voltage potential 126. Because of the way thediode 114 is connected in the first switching network 104, it is at thispoint reverse biased. Thus, no current will flow through the firstswitching network 104.

[0019] When the current flow through the solenoid coils 102 reaches afirst predetermined current magnitude, as indicated by the voltage dropacross the second resistor 122 reaching a first predetermined voltagemagnitude, the controller circuit 108 causes the first controllableswitch 110 to remain closed and causes the second controllable switch120 to open. The current in the solenoid coils 102, due to inductiveaction, will now flow through the first switching network 104.Specifically, current will flow from the solenoid coils 102, through thediode 114 and first controllable switch 110, back to the solenoid coils102. Once the current flow through the solenoid coils 102 decays to asecond predetermined current magnitude, the controller circuit 108causes the second controllable switch 120 to once again close.

[0020] The controller circuit 108 continues to operate the first 110 andsecond 120 controllable switches in the above-described current limitcycle until the first input port 128 receives the appropriate commandsignal for de-energizing the solenoid coils 102. When this occurs, thecontroller circuit 108 causes both the first 110 and second 120controllable switches to open. The current in the solenoid coils 102will now flow through the diode 114 and the first resistive element 112.The resistance and power rating of the first resistive element 112 areselected such that the energy in the solenoid coils 102 is quicklydissipated as heat.

[0021] The solenoid control circuit 100 may be implemented using variousknown circuit components and configurations. Having described thesolenoid control circuit from a generalized block diagram aspect, adescription of a particular implementation will now be described, withreference to FIG. 2, in which like reference numerals refer to likecircuit portions of the circuit depicted in FIG. 1.

[0022] In the depicted implementation, the first controllable switchingnetwork 104, which is connected in parallel with the solenoid coils 102,includes the first resistive element 112, and the diode 114. The firstcontrollable switch, however, is implemented using a firstmetal-oxide-semiconductor field effect transistor (MOSFET) 210. Thefirst MOSFET 210 is, in the depicted embodiment, an N-channel MOSFET,with its source (S) electrically connected to the first circuit terminal116, its drain (D) electrically connected between the first resistiveelement 112 and the diode 114, and its gate (G) coupled to receive afirst signal from the controller circuit 108.

[0023] The second controllable switching network 106 includes the secondresistor 122 and, as with the first controllable switching network 104,uses a second MOSFET 220 as the second controllable switch. The secondMOSFET 220 is also an N-channel MOSFET, and its source (S) iselectrically connected to the second resistor 122, its drain (D) iselectrically connected to the diode 114, and its gate (G) iselectrically connected to receive a second signal from the controllercircuit 108.

[0024] It will be appreciated that either or both of the first 210 andsecond 220 MOSFETs could be implemented as P-channel MOSFETs, so long asthe circuit operating voltages are implemented to provide the correctbiasing voltage to achieve proper circuit operation. It will also beappreciated that the first and second controllable switches could beimplemented as other components including, but not limited to, JFETs,IGBTs, bipolar transistors, thyristors, and SCRs.

[0025] Turning now to the controller circuit 108, it can be seen that inthe depicted embodiment it includes five major circuit portions, asolenoid current monitoring circuit portion 202, a comparator circuitportion 204, an input circuit portion 206, and a driver circuit portion208. Each of these circuit portions will now be described in moredetail, beginning first with the solenoid current monitoring circuitportion 202.

[0026] The solenoid current monitoring circuit portion 202 includes anamplifier circuit 212 and a timing circuit 214. The amplifier circuit212 has at least a first input 213 terminal, a second input terminal215, and an output terminal 217. The first input terminal 213 iselectrically connected between the second MOSFET 220 and the secondresistor 122. The second input terminal 215 is electrically connected toa resistor R11, which in turn is electrically connected to a referencepotential 219. The output terminal 217 is electrically connected to thetiming circuit 214. A feedback resistor R15 is electrically connectedbetween the amplifier output terminal 217 and the second input terminal215, thereby configuring the amplifier circuit 212 as a genericnon-inverting type of amplifier. It will be appreciated that the circuitresistors R11 and R15 are selected to provide the appropriate amount ofgain to set the solenoid holding current level. It will be furtherappreciated that the amplifier circuit 212 may also be configured as aninverting amplifier circuit.

[0027] The timing circuit 214 is electrically connected to the output217 of the amplifier circuit 212, and includes a resistor R9 and acapacitor C11. Thus, the capacitor C11 charges and discharges throughresistor R9. In particular, when the second MOSFET 220 is on and currentis flowing through the second resistor 122, the voltage developed acrossthe second resistor 122 is amplified by the amplifier circuit 212 andsupplied to its output terminal 217. This voltage on the output terminal217 of the amplifier 212 charges the capacitor C11 through resistor R9.Conversely, when the second MOSFET 220 is off and no current flowsthrough the second resistor 122, capacitor C11 is discharged throughresistor R9. The reasoning for this will be more fully described furtherbelow. It is to be appreciated that the values of C11 and R9 areselected to provide the desired discharge time of capacitor C11 when thesecond MOSFET 220 is off, and may be any one of numerous desired valuesthat meet the operational goals of the solenoid control circuit 100.

[0028] The comparator circuit portion 204 is electrically coupled toreceive a reference voltage 205 having a predetermined reference voltagemagnitude, the voltage that the capacitor C11 is charged to, which isrepresentative of the current magnitude flowing through the solenoidcoils 102, and the command signal from the input circuit portion 206.Based on these inputs, the comparator circuit supplies a switch controlsignal to the driver circuit portion 208. To carry out this function,the comparator circuit portion 204 includes a comparator 222 and alogical AND circuit 224. The comparator 222 has a first input terminal223, a second input terminal 225, and an output terminal 227. The firstinput terminal 223 is electrically connected to the timing circuit 214and the second input terminal 225 is electrically connected to areference voltage terminal 226 that is coupled to receive the referencevoltage 205. The comparator 222 is configured such that it supplies alogical high signal (e.g., approximately +5VDC in the depictedembodiment) whenever the capacitor C11 is charged to a voltage magnitudethat is less than or equal to a first comparator voltage magnitude, oris charging from a magnitude that is less than or equal to the firstcomparator voltage magnitude toward the predetermined referenced voltagemagnitude. The comparator 222 supplies a logical low signal (e.g.,approximately 0VDC in the depicted embodiment) whenever the capacitorC11 is charged to a voltage magnitude that is greater than or equal tothe predetermined reference voltage magnitude or is discharging from avoltage magnitude that is greater than or equal to the predeterminedreference voltage magnitude toward the first comparator voltagemagnitude.

[0029] The logical AND circuit 224 includes a first input terminal 229,a second input terminal 231, and an output terminal 233. As is generallyknown, the logical AND circuit 224 supplies a logical high signal on itsoutput terminal 233 only when logical high signals are supplied to boththe first 229 and the second 231 input terminals.

[0030] The input circuit portion 206 includes two inverter circuits, afirst inverter circuit 252 and a second inverter circuit 254. The firstinverter circuit 252 is coupled to the first input port 128. As waspreviously noted, the first input port 128 is used to receive thecommand signal for either energizing or de-energizing the solenoid coils102. The first inverter circuit 252 exhibits a high impedance to thecircuit or component supplying the command signal, and logically invertsthe command signal. In other words, if the command signal is a logicalhigh signal, the first inverter circuit supplies a logical low signal,and vice-versa. The second inverter circuit 254, which also has a highinput impedance, inverts the signal output by the first inverter 252, tosupply the correct logic level signal to the driver circuit portion 206.

[0031] The driver circuit portion 208 includes one or more drivers thatare used to switch the first 210 and second 220 MOSFETs on and off, inresponse to the command signal received from the input circuit portion206 and the switch control signal received from the comparator circuitportion 204. In the depicted embodiment, the driver circuit portion 208uses an integrated circuit 262 to carry out this functionality. Oneexample of an integrated circuit that performs this function is known asan IR2110, manufactured by International Rectifier. It will beappreciated that other integrated circuits may be used to carry out thissame function, or the driver circuit portion 208 could be implementedusing discrete components instead of an integrated circuit.

[0032] Having described both a general and specific embodiment of thepresent invention from a structural standpoint, an operationaldescription of the solenoid control circuit 100 depicted in FIG. 2, willnow be provided. In doing so, reference should be made to FIGS. 2 and 3,as necessary.

[0033] Initially, at time to, it will be assumed that the solenoidcontrol circuit 100 is energized, and is receiving an appropriatecommand to de-energize the solenoid coils 102 (e.g., a logic lowsignal). Thus, both the first 210 and second 220 MOSFETs are off (e.g.,non-conducting). At time t₁, the command signal goes high. As a result,the driver circuit portion 208 supplies a logic high signal to the gateof the first MOSFET 210, causing it to turn on (e.g., conduct).Substantially simultaneous with this, the command signal is supplied tothe first input terminal 229 of the AND circuit 224. Because no currentis yet flowing through the solenoid coils 102, there is no voltage dropacross the second resistor 122, thus the output of the comparator 224,which is coupled to the second input 231 of the AND circuit 224 is alogic high. Since both inputs to the AND circuit 224 are high, itsoutput 223 is high. As a result, the driver circuit 208 supplies a logichigh signal to the gate of the second MOSFET 220, as well. This causescurrent to flow from the first voltage potential 118, through thesolenoid coils 102, the second MOSFET 220, and the second resistor 122.

[0034] At time t₂, the magnitude of the current flow through thesolenoid coils 102 has reached the first predetermined currentmagnitude. This is determined by the magnitude of the voltage dropacross the second resistor 122 reaching the first predetermined voltagemagnitude. As was noted above, the voltage drop across the secondresistor 122 is amplified and charges capacitor C11 to the predeterminedreference voltage magnitude. When this occurs, the output of thecomparator 222 goes low, the output of the AND circuit 224 goes low, andthe driver circuit 208 supplies a logic low signal to the gate of thesecond MOSFET 220, turning it off. When the second MOSFET 220 turns off,current no longer flows through the second resistor 122. Thus, thevoltage drop across the second resistor falls to zero. As a result,capacitor C11 starts discharging through R9. At substantially the sametime, current now flows through the solenoid coils 102, the diode 114,and the first MOSFET 210, and not the first resistive element 112.

[0035] Capacitor C11 continues to discharge until time t₃, when thepotential across it reaches the above-noted first comparator voltagemagnitude. It is noted that the values of capacitor C11 and resistor R9are chosen such that the magnitude of the current flowing through thesolenoid coils 102 has decayed to at least the second predeterminedcurrent magnitude in the time it takes for capacitor C11 to dischargefrom the predetermined referenced voltage magnitude to the firstcomparator voltage magnitude. At this time, the outputs of thecomparator 222 and the AND circuit 224 once again go high, causing thedriver circuit 208 to supply a logic high signal to the gate of thesecond MOSFET 220. Thus, the second MOSFET 220 once again conducts.

[0036] This current limiting cycle continues until time t₉, when thecommand signal goes low. As a result, the driver circuit 208 supplies alogic low signal to the gate of the first MOSFET 210, causing it to turnoff. Substantially simultaneous with this, the logic low signal issupplied to the first input terminal 229 of the AND circuit 224, causingits output 227 to be low. In response, the driver circuit 208 supplies alogic low signal to the gate of the second MOSFET 220, turning it off aswell. At this point, current flows through the solenoid coils 102 andthe first resistive element 112. With the solenoid coil 102 currentflyback path now including first resistive element 112, the energystored in the solenoid coils 102 is quickly dissipated, which in turnspeeds up the turn off time of the solenoid 102.

[0037] With the solenoid control circuit and method of the presentinvention, current flows through the first resistive element 112 onlywhen the controller circuit 108 receives an appropriate command forde-energizing the solenoid coils 102, and not during the current limitcycle. Thus, the energy dissipated by the first resistive element 112 isreduced relative to other solenoid driver circuits, which makes it moreefficient compared to other circuits, and EMI emissions from the circuit100 are also reduced compared to other circuits.

[0038] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

I claim:
 1. A circuit for controlling current flow through a solenoid, comprising: a first controllable switch electrically coupled in parallel with a first resistive element; a second controllable switch electrically coupled in series with a second resistor; a diode electrically coupled in series between the first controllable switch and the second controllable switch; and a controller circuit operable to (i) selectively open and close the first controllable switch and the second controllable switch in response to a command signal and (ii) selectively open and close the second controllable switch based on a magnitude of current flow through the solenoid.
 2. The circuit of claim 1, wherein the controller circuit comprises: a comparator circuit having at least a first input terminal coupled to a reference voltage, a second input terminal coupled to receive a voltage signal representative of a current magnitude flowing through the solenoid, and a third input terminal coupled to receive the command signal, the comparator operable to supply a switch control signal based on the signals on the first, second, and third input terminals; and a driver circuit having at least a first input coupled to receive the switch control signal and operable, in response thereto, to selectively open and close the second controllable switch.
 3. The circuit of claim 2, wherein the driver circuit further has at least a second input coupled to receive the command signal and operable, in response thereto, to selectively open and close the first controllable switch and the second controllable switch.
 4. The circuit of claim 3, wherein the controller circuit further comprises an input circuit operable to condition the command signal and supply the conditioned command signal to the driver circuit second input and the comparator circuit third input.
 5. The circuit of claim 2, wherein the comparator circuit comprises: a comparator having a first comparator input terminal electrically coupled to the comparator circuit first input terminal and a second comparator input terminal electrically coupled to the comparator circuit second input terminal, and a comparator output terminal; and a logical AND circuit having a first AND circuit input coupled to the third comparator circuit input and a second AND circuit input coupled to the comparator output terminal and operable, in response thereto, to supply the switch control signal.
 6. The circuit of claim 2, wherein the controller circuit further comprises: a solenoid current monitoring circuit electrically coupled to receive a signal representative of current magnitude flowing through the solenoid and operable, in response thereto, to supply the voltage signal representative of a current magnitude flowing through the solenoid.
 7. The circuit of claim 6, wherein the solenoid current monitoring circuit comprises: an amplifier circuit electrically coupled to receive the signal representative of the current magnitude flowing through the solenoid and operable to provide an amplified current magnitude signal; and a timing circuit coupled to receive the amplified current magnitude signal and operable to supply the a voltage signal representative of a current magnitude flowing through the solenoid.
 8. The circuit of claim 1, wherein the controller supplies a first switch control signal to cause the first controllable switch to selectively open and close and a second switch control signal to cause the second controllable switch to open and close.
 9. The circuit of claim 8, wherein: the first controllable switch comprises a first MOSFET having at least a gate terminal electrically coupled to receive the first switch control signal, a source terminal electrically coupled to one terminal of the first resistive element, and a drain terminal electrically coupled to another terminal of the first resistive element; and the second controllable switch comprises a second MOSFET having at least a gate terminal electrically coupled to receive the second switch control signal, a source terminal electrically coupled to one terminal of the second resistor, and a drain terminal electrically coupled to a terminal of the diode.
 10. The circuit of claim 1, wherein: current flows through the solenoid, the second controllable switch, and the second resistor when the first controllable switch and the second controllable switch are closed; and current flows through the solenoid, the diode, and the first controllable switch when the first controllable switch is closed and the second controllable switch is open.
 11. The circuit of claim 10, wherein current flows through the solenoid, the diode, and the first resistive element when the first and second controllable switches are open after the being closed simultaneously for a predetermined time period.
 12. The circuit of claim 1, further comprising: a solenoid coupled electrically coupled in parallel with the first controllable switch and the diode.
 13. A circuit for controlling current flow through a solenoid, comprising: a diode having at least a first diode terminal and a second diode terminal; a first controllable switch electrically coupled in series between the first diode terminal and a first circuit terminal, the first circuit terminal adapted for coupling the circuit to a first predetermined voltage magnitude; a first resistive element having at least a first resistive element terminal and a second resistor terminal, the second resistor terminal electrically coupled to a second circuit terminal that is adapted for coupling the circuit to a second predetermined voltage magnitude; a second controllable switch electrically coupled in series between the second diode terminal and the first resistive element terminal; and a controller circuit having at least a first input coupled to receive a command signal and operable to (i) selectively open and close the first controllable switch and the second controllable switch in response to the command signal and (ii) selectively open and close the second controllable switch based on a magnitude of current flowing through the solenoid.
 14. The circuit of claim 13, wherein the controller circuit comprises: a comparator circuit having at least a first input terminal coupled to a reference voltage, a second input terminal coupled to receive a voltage signal representative of a current magnitude flowing through the solenoid, and a third input terminal coupled to receive the command signal, the comparator operable to supply a switch control signal based on the signals on the first, second, and third input terminals; and a driver circuit having at least a first input coupled to receive the switch control signal and operable, in response thereto, to selectively open and close the second controllable switch.
 15. The circuit of claim 14, wherein the driver circuit further has at least a second input coupled to receive the command signal and operable, in response thereto, to selectively open and close the first controllable switch and the second controllable switch.
 16. The circuit of claim 15, wherein the controller circuit further comprises an input circuit operable to condition the command signal and supply the conditioned command signal to the driver circuit second input and the comparator circuit third input.
 17. The circuit of claim 14, wherein the comparator circuit comprises: a comparator having a first comparator input terminal electrically coupled to the comparator circuit first input terminal and a second comparator input terminal electrically coupled to the comparator circuit second input terminal, and a comparator output terminal; and a logical AND circuit having a first AND circuit input coupled to the third comparator circuit input and a second AND circuit input coupled to the comparator output terminal and operable, in response thereto, to supply the switch control signal.
 18. The circuit of claim 14, wherein the controller circuit further comprises: a solenoid current monitoring circuit electrically coupled to receive a signal representative of current magnitude flowing through the solenoid and operable, in response thereto, to supply the voltage signal representative of the current magnitude flowing through the solenoid.
 19. The circuit of claim 18, wherein the solenoid current monitoring circuit comprises: an amplifier circuit electrically coupled to receive the signal representative of the current magnitude flowing through the solenoid and operable to provide an amplified current magnitude signal; and a timing circuit coupled to receive the amplified current magnitude signal and operable to supply the voltage signal representative of the current magnitude flowing through the solenoid.
 20. The circuit of claim 14, wherein the controller supplies a first switch control signal to cause the first controllable switch to selectively open and close and a second switch control signal to cause the second controllable switch to open and close.
 21. The circuit of claim 20, wherein: the first controllable switch comprises a first MOSFET having at least a gate terminal electrically coupled to receive the first switch control signal, a source terminal electrically coupled to one terminal of the first resistive element, and a drain terminal electrically coupled to another terminal of the first resistive element; and the second controllable switch comprises a second MOSFET having at least a gate terminal electrically coupled to receive the second switch control signal, a source terminal electrically coupled to one terminal of the second resistor, and a drain terminal electrically coupled to a terminal of the diode.
 22. The circuit of claim 13, wherein: the current flowing through the solenoid flows through the second controllable switch and the second resistor when the first controllable switch and the second controllable switch are closed; and the current flowing through the solenoid, flows through the diode and the first controllable switch when the first controllable switch is closed and the second controllable switch is open.
 23. The circuit of claim 22, wherein the current flowing through the solenoid, flows through the diode and the first resistive element when the first and second controllable switches are open after the being closed simultaneously for a predetermined time period.
 24. The circuit of claim 13, further comprising: a solenoid coupled electrically coupled in parallel with the first controllable switch and the diode.
 25. A circuit for controlling current flow through a solenoid, comprising: a first MOSFET having at least a gate terminal, a source terminal, and a drain terminal; a first resistive element having a first terminal coupled to the first MOSFET source terminal and a second terminal electrically coupled to the first MOSFET drain terminal; a second MOSFET having at least a gate terminal, a source terminal, and a drain terminal; a second resistor having a first terminal coupled to the second MOSFET source terminal; a diode electrically coupled in series between the first MOSFET drain terminal and the second MOSFET source terminal; and a comparator circuit having at least a first input terminal coupled to a reference voltage, a second input terminal coupled to receive a voltage signal representative of a current flowing through the solenoid, and a third input terminal coupled to receive the command signal, the comparator operable to supply a switch control signal based on the signals on the first, second, and third input terminals; a solenoid current monitoring circuit electrically coupled to receive a signal representative of a magnitude of current flowing through the solenoid and operable, in response thereto, to supply the voltage signal representative of the current flowing through the solenoid; and a driver circuit having at least a first input coupled to receive a command signal and a second input coupled to receive the switch control signal and operable, the driver circuit operable to selectively supply a first gate signal to the first MOSFET gate terminal in response to the command signal and to selectively supply a gate signal to the second MOSFET gate terminal in response to the switch control signal.
 26. In a circuit including series connected first controllable switch and a diode that are electrically coupled in parallel with a solenoid, and a series-connected second controllable switch and first resistor that are electrically coupled in series with the solenoid, a method of controlling current flow through the solenoid, comprising: closing the first and second controllable switches, whereby a current flows from a first voltage potential through the solenoid, the second controllable switch, and the first resistor; determining when the current flow through the solenoid reaches a first predetermined current magnitude; and opening the second controllable switch when the first predetermined current magnitude is reached, whereby the current flows through the solenoid, the diode, and the first controllable switch.
 27. The method of claim 26, further comprising: after opening the second controllable switch, determining when the current flow through the solenoid reaches at least a second predetermined current magnitude, that is less than the first predetermined magnitude; and closing the second controllable switch when the second predetermined current magnitude is reached, whereby the current flows from the first voltage potential through the solenoid, the second controllable switch, and the first resistor.
 28. The method of claim 26, wherein the step of determining when the current flow through the solenoid reaches the first predetermined current magnitude comprises: determining when a voltage drop across the first resistor reaches a first predetermined voltage magnitude.
 29. The method of claim 27, wherein the step of determining when the current flow through the solenoid reaches the second predetermined current magnitude comprises: determining that the second controllable switch has been opened for a predetermined time period, such that the current flow through the solenoid has decayed to at least the second predetermined current magnitude.
 30. The method of claim 26, further comprising: opening the first and second controllable switches, whereby the current flow through the solenoid is dissipated by a second resistor that is electrically coupled in parallel with the first controllable switch. 